1. Field of the Invention
The present invention relates generally to a proportioning valve assembly for adjusting supply pressure of fluid, such as hydraulic working fluid and so forth, to a work so as to adjust the fluid pressure in the work. More specifically, the invention relates to a proportioning valve assembly which is applicable for a work which requires fluid pressure adjustment frequently with high response characteristics to adjust for pressure variation. The present invention also relates to an actively controlled suspension system which is disposed between a vehicular body and a vehicular wheel and is adjustable of suspension characteristics for regulating vehicular body height level and for suppressing attitude change of the vehicular body.
2. Description of the Background Art
Co-pending U.S. patent applications propose actively controlled suspension systems which generally achieve vehicular height regulation for maintaining vehicular height level at a predetermined height range and, as well, achieve attitude change suppression for better driving stability and riding comfort. Such actively controlled suspension systems employ proportioning valve assemblies in a pressure circuit for adjusting fluid pressure in working chambers defined within hydraulic cylinders disposed between vehicular bodies and suspension members rotatably supporting vehicular wheels.
Generally, the proportioning valve assembly employed in co-pending U.S. patent applications, is disposed within a hydraulic circuit connecting a pressurized working fluid source unit and the working chamber. The proportioning valve assembly comprises a three way spool valve or poppet valve which is supplied substantially constant line pressure from the pressurized working fluid source and output adjusting pressure is controlled by an electric actuator, such as an electrically driven proportioning solenoid and so forth. The actuator operates to adjust a pilot pressure to be exerted on the pressure controlling valve, such as the spool valve or poppet valve. A pilot chamber in which the pilot pressure is generated, is selectively connected to a control port connected to the working chamber of the hydraulic cylinder to receive therefrom the control pressure to be supplied to the working chamber as feedback pressure, and to a drain port for draining pressure to the pressurized working fluid source via a drain line.
In the pressure circuit between the proportioning valve assembly and the pressurized working fluid source, part of line pressure is directly fed back from a supply line to a drain line. This tends to generate back pressure in the drain line. When the pilot chamber is connected to the drain port for draining the pressurized fluid for reducing the pilot pressure, the back pressure tends to influence to the pressure in the pilot chamber and thus disturb pilot pressure. In order to avoid influence of the back pressure in the drain line, it is general technology to provide an orifice to restrict fluid flow therethrough.
The orifice in the known art comprises a singular projection projecting into the fluid path for reducing fluid flow path area and thereby increasing flow resistance. In case of the actively controlled suspension system, providing a singular orifice for preventing the influence of the back pressure in the drain line was not satisfactorily effective for stabilizing the pressure in the pilot chamber. Furthermore, flow restriction by means of the orifice causes lowering of response characteristics in adjustment of the pilot pressure. Because quick and high level response ability is required for the proportioning valve assembly employed in the actively controlled suspension system, the providing of an orifice creates a defect in the system in view of lowering of the response characteristics.
On the other hand, the proportioning valve assembly in the actively controlled suspension system is generally designed for absorbing road shock transmitted from the vehicular wheel. In the road shock absorbing operation, fluid pressure in the working chamber is drained through the proportioning valve in response to bounding shock for absorbing the bounding energy and is supplied through the proportioning valve in response to rebounding shock for absorbing rebounding energy. In order to achieve satisfactory road shock absorption, it is required to permit relatively large amount of fluid flow. In order to accomplish large amount of fluid supply for the working chamber in response to the bounding shock of a frequency close to vehicular body resonating frequency, a large pressurized working fluid supply capacity is required for the pressurized working fluid source unit. Since the pressurized fluid source unit generally comprises an engine driven fluid pump associated with an automotive engine to be driven by the output torque of the engine, a greater capacity pump to be employed in the pressurized working fluid source unit may cause greater load on the engine and increase power loss.